This invention relates to solid state circuit breakers for controlling the supply of power from an AC supply to a load and particularly to such circuit breakers suitable for application in three-phase systems.
Copending application Ser. No. 023,361, filed Mar. 23, 1979, by Billings and Luebrecht and assigned to the present assignee (Case W.E. 47,975) discloses an AC solid state circuit breaker achieving the multiple objectives of good simulation of electromechanical circuit breaker characteristics, including the provision of a manual control switch for toggling between off, on and reset conditions, while doing so with apparatus whose size and weight are minimized and reliability is maximized. The disclosure of the copending application is incorporated herein by reference including the background material referenced therein.
In the apparatus of the copending application there is provided means for providing a power removal feature for load power by locating a manual control switch so that it opens essentially all of the circuit except the static power switch branch between the source and the load. This provides a high degree of failsafe protection for the apparatus. A power continuity feature is provided by means for zero crossover synchronization both upon power up and power down conditions. A circuit lock out feature is used so that on power up or turn on, operation is delayed until the occurrence of an internal power ready condition. On power down or turn off, there is means for circuit retention subsequent to switch opening and internal power removal in order to accomplish load turn off at a desired zero crossover point of the load power through the power switch. In addressing the requirements for a solid state circuit breaker to operate in a three phase system, or some other number of a plurality of phases, there are additional complications addressed by the present invention in order to assure effective circuit breaker operation including the power continuity feature.
In accordance with the present invention, a solid state circuit breaker is provided with circuitry for permitting off/on/reset operations actuated by a mechanical toggle switch for close simulation of magnetic circuit breaker operation in three phase or other multiphase systems. The power continuity feature is provided for load power by locating the control switch so that it opens essentially all of the circuit except the static power switch branch. The control switch is located between two of the supply voltage phases and the power supply for the control circuitry. Zero crossover synchronization means is provided to make sure the power on and power off events occur at optimum points in the applied AC voltage waveform in order to reduce transient and saturation effects in the loads on the circuit. For example, a straightforward application of known power controller techniques for three phase apparatus would entail means for turning on each phase separately, at 120.degree. intervals, at its correct zero crossover point. This would require the use of three zero crossover circuits and also associated apparatus including the oscillators for the drive circuits of the AC switches. In so doing, there would be a substantial increase in the size and cost of the circuit breaker that would be undesirable. In accordance with this invention, the correct zero crossover of one phase is utilized to produce turn-on at zero degrees of that one phase while the other two phases are respectively at minus 120.degree. and plus 120.degree. without any significant impairment of overall circuit breaker performance.
Additionally, apparatus in accordance with this invention can be made for three phase operaton over any of a wide range of frequencies (such as 60 Hz to 400 Hz) while at the same time preserving the capability to be operated in a single phase system so that a single solid state circuit breaker can be manufactured and supplied that permits a wide range of applications by the user.
In the course of achieving the primary purposes of the present invention, there have been encountered specific improvements in parts of the overall power controller or circuit breaker that are generally applicable in such solid state apparatus. In particular, an overcurrent protection system is included that utilized optical isolators with reduced sensing voltages developed directly from terminals of the power switch elements that has been found generally useful and highly efficient.